Both wind turbines, planes and helicopters are using airfoils which need to have a high lift coefficient and good lift/drag performance. It is state of the art to apply vortex generators to existing airfoils since it is known that this will increase the maximum lift coefficient. However even after much research efforts, the use of vortex generators has not shown a breakthrough development since it is associated with the following problems.
The performance of an airfoil can be expressed by the lift coefficient and the lift over drag ratio. The precise required values depend on the technical application of the airfoil, however in general, when the application of vortex generators is considered, the designer has an airfoil without vortex generators which performs well up to a certain angle of attack (αNVG), and wants to extend the good performance range beyond αNVG by using vortex generators. This can be realized with classical vortex generators, however, for angle of attack below αNVG, the lift over drag ratio decreases substantially. This is usually not acceptable. For example in WO90/11929 by Wheeler a large number of geometries is proposed. This publication does not disclose which geometry would give acceptable performance. It was shown that the geometries revealed by Wheeler in FIGS. 1 and 2 and 4 did not provide airfoil performance which was better than that of airfoils without vortex generators. Also WO00/15961 discloses several geometries which are shown not to give acceptable performance.
A second problem is that the attachment of vortex generators often fails. Vortex generators usually exist of baseplates with one or more fins which are glued on the airfoil surface. In the practice of wind turbines the vortex generators come loose within weeks or a few years. It was tried to increase the size of the base so that the attachment area was increased. This idea did not give better fixation. For example in EP 2031243A1 by LM a radical solution was proposed by submerging vortex generators in the airfoil surface. However it should be noted that airfoil have to pass through high bending moments, which means that the surface experiences high stress and thus that a discontinuity in the surface such as is proposed by LM is unacceptable. Another attempt was to produce strips with multiple vortex generator pairs so that many vg-fins could be applied quickly and the full strip surface was fixed by adhesive. Strips of plastic and of aluminium have been applied on many rotor blades but came loose within a few weeks-years. Also the fixation of the strips with high quality silicone based adhesive did not improve the situation.
A third problem is that the vortex generator should be resistant to all possible weather conditions, which also sets shape demands. For example a thickness of 2-5 mm or more in particular 3-4 mm is required for standard plastics to have long term UV-resistance. Such thickness cause the vortex generator to become stiff so that it cannot adapt to the shape of the airfoil surface. Furthermore such thickness for the baseplate of the vortex generator means that the flow has to step up and step down the baseplate over at least the 2-5 mm and it means that the vortex generator fins need to be relatively thick which all leads to poor aerodynamic performance. Another attempt to solve this problem was to place the vg-strips in a recess in the blade surface. The aim was to reduce the aerodynamic drag caused by the strips when place on top of the surface. In practice, however, it was not feasible to produce the recess and the vg-strips so accurately that, the blade surface was smooth with the vg-strips positioned in the recess. Furthermore, these vgs did come loose quickly and the recess in the blade surface reduced building height and caused cracks in the surface.
A fourth problem is that the vortex generator should be easily applicable and should be shaped such that objects would not become stuck by the vortex generator fins and should not be intrusive in the airfoil. In the practice of wind energy often triangular shaped vortex generator fins are applied since this shape is proven to generate strong vortices, however, the sharp edges could injure service personnel and obstruct hoisting belts. It is clear that this problem also sets demands on the shape so that it becomes even more difficult to obtain an aerodynamically optimized shape. It is known from fundamental theoretical analysis and not doubted that strong vortices are generated when a bound aerodynamic circulation suddenly stops. This is why the sharp-edge triangular form and also a sharp-edge rectangular form cannot be avoided when strong concentrated vortices are required. So the expert in the art has not much room to solve the problems.
In the light of the above problems, it needs explanation why vortex generators are often applied to stall regulated wind turbines. The reason is that many stall regulated turbines suffered from large underperformance and that the application of vortex generators could solve this to a large extent. However the solution was not durable since the vortex generators came loose. Therefore, designers of wind turbines put effort in new blade and airfoil designs which did not require vortex generators. This design method was rather successful and therefore wind turbines typically do not have have vortex generators.